Types of optocouplers
Optical couplers are passive devices that split, combine, and distribute optical signals. They are indispensable optical components in wavelength division multiplexing, fiber optic local area networks, fiber optic cable television networks, and certain measuring instruments. Several typical fiber optic coupler structures are shown in the figure.
Working principle
A 4-port optocoupler is the simplest type of device. The structure and principle of a 4-port optocoupler are shown in the figure.
Performance parameters
(1) Insertion Loss
Insertion loss refers to the ratio of the optical power at a specific port at the input end to the optical power at another port at the output end after light passes through the device. The insertion loss from the input port to the output port is expressed as
L_i = 10 log (P_out / P_in) (3-31)
(2) Additional Loss
Additional loss L_a is defined as the ratio of the total input power to the total output power. As shown in Equation 3-32 for a 4-port optical coupler,
L_a = 10 log (P_in / (P_1 + P_2)) (3-32)
(3) Splitting Ratio
The splitting ratio is a percentage that indicates the ratio of the optical power output from one port to the total optical power output from all ports. It reflects the proportion of power distribution at the output ports. For a 4-port optical coupler, it can be expressed as
S_n = (P_2 / (P_1 + P_2)) × 100% (3-33)
(4) Isolation
Isolation refers to the ability to block or attenuate the optical path between non-connected ports. It indicates that the power output at the desired output port is much greater than that at the undesired output ports. For a 4-port optical coupler, its mathematical expression is
L_g = -10 log (P_2 / P_in) (3-34)
The physical structure diagram of the three-port optical coupler is shown in the figure.
Optical Isolators and Optical Circulators
Optical Isolator
The function of an optical isolator is to ensure that light waves can only propagate in the forward direction, preventing reflected light caused by various factors in the transmission line from re-entering the laser and affecting the laser's operational stability.
Optical isolators are primarily used after lasers or optical amplifiers. Lasers and optical amplifiers are very sensitive to reflected light from connectors, splices, and filters. This reflected light can degrade their performance; for example, the spectral width of a laser can be broadened or narrowed by the reflected light, sometimes by several orders of magnitude. Therefore, an optical isolator should be placed near the output of such optical devices to prevent the effects of reflected light.
The main performance indicators of an optical isolator include operating wavelength, typical insertion loss (reference value: 0.4 dB), maximum insertion loss (reference value: 0.6 dB), typical peak isolation, minimum isolation (reference value: 40 dB), and return loss (i.e., reflection loss, reference value: input/output 60/60 dB), etc.
Optical circulator
Optical circulators and optical isolators operate on essentially the same principle, except that optical isolators are generally two-port devices, while optical circulators are multi-port devices. Optical circulators are important components in bidirectional communication, as they can separate forward and reverse transmitted light, and are used in single-fiber bidirectional communication. A schematic diagram of an optical circulator is shown on the left, and a schematic diagram of an optical circulator used in single-fiber bidirectional communication is shown on the right.
Wavelength converter
A wavelength converter is a device that converts a signal from one wavelength to another. Wavelength converters can be classified into optoelectronic wavelength converters and all-optical wavelength converters based on their wavelength conversion mechanism.
The optoelectronic wavelength converter is shown in the figure. Due to speed limitations imposed by electronic devices, it is not suitable for high-speed, high-capacity fiber optic communication systems.
The all-optical wavelength converter is shown in Figure 3-38. Its wavelength conversion technology mainly consists of a semiconductor optical amplifier (SOA).
A light signal with wavelength λ₁ and a continuous light signal with wavelength λ₂ are simultaneously fed into a semiconductor optical amplifier (SOA). The SOA exhibits gain saturation characteristics with respect to the input optical power. As a result, the information carried by the input light signal is transferred to λ₂, and by extracting the λ₂ light signal through a filter, all-optical wavelength conversion from λ₁ to λ₂ can be achieved.